app.js

var express = require('express');
var app = express();

var gcloud = require('gcloud')({ /* extra config here */ });
var dataset = gcloud.datastore.dataset({ /* projectId: my-project */ });

// Set the trace ID on a per-request basis via middleware
// (See http://expressjs.com/guide/writing-middleware.html)
app.use(function(req, res, next) {
  // Need to double check that this works from a thread safety perspective.
  gcloud.setTraceContext(req.headers['X-Cloud-Trace-Context']);
});

// Get companies from the datastore.
app.get('/company/:id', function(req, res, next) {
  var key = dataset.Key(['Company', req.params.id]);
  // This request will have the trace ID that was set by the middleware.
  // So whatever was in the request header will be automatically forwarded along.
  dataset.get(key).on('data', function(entity) {
    res.send(entity);
  });
});

In that example, bucket.getMetadata() would not know anything about the decorated request options. You would have to access the bucket from threadLocalgcloud.bucket() to have that stuff on there.

In a practical use, if I need to decorate all requests, I would do this:

var gcloud = require('gcloud')({
  decorateRequest: function(reqOpts) {
    reqOpts.headers['A'] = 1;
    reqOpts.headers['B'] = 1;
    return reqOpts;
  }
});